The present invention is generally related to marine propulsion systems, and, more particularly, the present invention is related to through gimbal exhaust assemblies and techniques that may be used in a stern drive marine propulsion system.
Present technology for exhaust systems in stem drives either routes exhaust overboard through the transom or through the propeller underwater. Through prop exhaust is somewhat quieter relative to overboard exhaust systems since the exit is underwater and generally far from the boat interior and the exhaust is within the vortex of the propeller. FIG. 2 shows a cross-sectional view of a typical prior art exhaust system arrangement that uses a flexible bellows 50 for passing exhaust from an internal combustion engine to a passageway in a propulsion unit for discharge through the propeller. The exhaust may typically comprise exhaust gases and cooling water to avoid high temperature conditions which could burn the bellows. As shown in FIG. 2, bellows 50 comprises a plurality of circumvolutions that allows bellows 50 to be flexibly extended for allowing respective pivotal motion of propulsion unit 14 about a generally vertical steering axis, and about a generally horizontal tilt/trim axis. As further shown in FIG. 2, a separate flexible bellows 52 encloses a universal joint 54 that allows for transmitting rotating power from the engine to the propulsion unit in fashion well-understood by those skilled in the art. It will be appreciated that the exhaust bellows arrangement should be reliable as such arrangement allows for containing the exhaust and noise during various trim and/or steering conditions. This prior art arrangement works generally satisfactory for most conditions. However, during periods of high volume of exhaust flow, such as during periods of high engine load, the circumvolutions in the interior of the bellows, as represented by the curls in the interior of bellows 50, may result in an undesirably high level of friction in the exhaust that flows in the bellows. The high friction in turn may cause a relatively high level of exhaust back pressure which results in reduced engine efficiency.
Manufacturers of marine propulsion systems have attempted to solve the foregoing issues but some of these attempts may have their own side effects. For example, the following two prior art arrangements may have somewhat helped to reduce back pressure but each is believed to suffer from increased exhaust noise or exhaust leakage, or both. One of such exhaust arrangements allows for providing transom exhaust relief holes for discharging exhaust ahead of the bellows. The other prior art exhaust arrangement, once used but abandoned because of its side effects, substituted two straight slip-together rubber tubes, in lieu of a flexible bellows, to carry the exhaust. Unfortunately, such arrangement, like the one with exhaust relief holes, resulted in leaking exhaust and noise during various trim and steering conditions.
In view of the foregoing discussion, it is desirable to provide a gimbal assembly that can be produced and maintained at a low cost and that avoids such side effects while reducing exhaust flow friction so as to reduce exhaust back pressure and achieve high engine efficiency without creating loud exhaust noise during high engine load, or annoying "burping" noises during engine idling conditions.